Parabolic reflector antennas are widely used as satellite television antenna due to a number of factors like the following:
low cost;
wide frequency range;
simplicity of working with waves of different polarization;
reasonable high aperture efficiency (AE)—usually 60-65%.
There is a known device such as axially symmetric dual reflector antenna with offset from symmetry axis main reflector focus (Patent Great Britain No. 973583, HO1D, published 1962). In this design, a parabolic shaped main reflector and an arbitrary shaped sub-reflector are used. As a particular case, an elliptically shaped sub-reflector is offered. The arrangement of the sub-reflector focus, the main reflector focus and feed phase center is common, i.e. first focus of the ellipse coincides with phase center and second focus of the ellipse coincides with focus of the parabola.
There is a known device as an antenna where focuses of a parabolic main reflector and a sub-reflector are displaced so that the sub-reflector vertex and above mentioned focuses are disposed on one straight line and the ratio of focal diameters of the sub-reflector and the main reflector is chosen in range of 1.03-1.07 (Patent USSR No. 588863, H01Q15/00, published in 1972).
In this design, a problem for antenna gain increasing is solved and the antenna itself suffers from large lateral size and especially large longitudinal size.
In another known patent (Patent USSR No. 1804673, H01Q19/18, published 1993), it is mentioned that radiating horn radiates not perfectly spherical wave but a wave with diffused center. Owing to this fact included in the above patent, phase error is corrected by the shape of a sub-reflector further comprising one focus coinciding with a parabolic main reflector focus.
Typically, parabolic antennas occupy a large volume. Most advantages of parabolic antennas appear when the ratio of antenna focal length F and antenna diameter D is sufficiently large. As antenna feed must be certainly placed in the reflector focus, it necessarily leads to the increase of the antenna system size.
Large system size leads to the following disadvantages:
A great number of such antennas disfigures architectural image of buildings. In particular, many countries prohibit installation of parabolic antennas or walls and roofs for this reason.
Parabolic antennas are impossible or very difficult to use in mobile devices, especially when required to provide signal reception during the movement of a car, train, ship, etc.
Due to the above mentioned circumstances, an actual problem arises—to develop for satellite TV or any other flat antennas which occupy sufficiently less volume.
The feature of dual reflector antennas with minimal thickness is that their radiator horns and sub-reflectors form an electromagnetic field which differs from geometrical optics field. Therefore, the choice of antenna parameters claimed in the patents mentioned above is not optimal neither is it applicable to the problem at hand. This statement is verified by U.S. Pat. No. 6,603,437 which claims an algorithm for shape choice of a main reflector and a sub-reflector which gives an optimal solution only for the sub-reflectors of diameter not less than five free-space wavelengths.
In case of antennas with minimal thickness and maximal aperture efficiency, the above mentioned condition may not be correct at least for antennas having a main reflector diameter less than 36 wavelengths. It is obvious that usage of big electrical size sub-reflectors will lead to aperture efficiency decrease due to the shadowing of the main reflector by sub-reflector. As an example, therefore, maximal values of aperture efficiency are achieved when sub-reflector diameter is about 2-3 wavelengths. Note that antenna thickness is from 1 to 3.5 wavelength when its main reflector diameter is from 5 to 18 wavelength. At such sizes of radiator horns and sub-reflectors, their focuses are diffused and incident to the main reflector thus wave beam forming can not be described correctly in terms of geometrical optics.
There is a known technical solution in which suggests to connect dual polarized antennas by means of dual mode waveguides. For instance, circular or square (U.S. Pat. No. 5,243,357). The width of dual mode waveguide must not be less than 0.5 wavelength. Single mode waveguide may have thickness much smaller than 0.5 wavelength. Real lateral dimension size of a dual mode waveguide is about 0.7 wavelengths. Therefore, Connection of some units of antennas into one antenna array based on dual mode waveguides can not be thinner than above mentioned 0.7 wavelengths. Waveguide bends which necessarily appear in such connections, should be added to this value. Thus, the real thickness of such connection will not be less than 1.5 wavelength. Furthermore, dual mode waveguide components produce hard requirements to waveguide elements manufacturing accuracy because technological errors may lead to differently polarized waves interconnection which will downgrade the device parameters.
As an example, an antenna-feeder device comprises four dual reflector antennas positioned in one plane, a main reflector of each antenna is formed by parabolic generatrix rotation around an axis, where focus of parabolic generatrix is situated outward from rotation axis, and a sub-reflector is formed by elliptic generatrix rotation around the same axis with forming of circle and vertex faced to the main reflector and situated between the circle and the main reflector, where one of the elliptic generated focuses is situated on the rotation axis, and radiators for each antenna are situated on the rotation axis in the main reflector base between the parabolic surface main reflector and the sub-reflector, feeding device is made on the base of dividers, where each of dividers is made as a junction of single mode transmission lines and each of dividers is made with equi-phase power division on two equal halves, input of feeding device can be connected with receiving and/or transmitting device, and four outputs of feeding devices are correspondingly connected with antenna radiators (Japanese Patent JP61245605, H 01 Q 21/06, published Oct. 31, 1986.
This device can not provide antenna operation on two orthogonal polarizations, and only single polarization work is provided. The limitations of this technical solution also include large lateral and transversal dimensions.
The present invention provides an antenna-feeder device and antenna with smaller size than current solutions.
Some of the technical advantages that may be achieved by manufacturing an antenna-feeder device and antenna in accordance with preferred embodiments of the present invention are reduction of device/antenna size and thickness, providing possibility of transmitting/receiving signals of both orthogonal polarizations with high isolation—not less than 20 dB, while covering a broad frequency range. By way of example a well designed antenna according to the preferred embodiments may cover the entire satellite TV range of 10.7-12.75 Ghz. Clearly other ranges of frequencies are achievable as will be clear to the skilled in the art.
Yet another desired technical result that may be achieved by the antenna-feeder device and antenna is reducing of longitudinal size with retention of high aperture efficiency and wide frequency range.
In these specifications, the term “circle” denotes a circle, formed by the intersection of a body of rotation formed when a parabolic or elliptic shape is rotated about an axis of rotation, and a plane perpendicular to the axis of rotation. It is notable that while the description and the claims utilize to the geometrical form, engineering considerations may dictate deviation from this ideal shape, yet allow a functionally equivalent shape to perform in accordance with the mode of operation and the functions described herein, and thus the invention and the claims should be construed to extend to such embodiments.